1. Field of the Invention
The present invention relates to an adjusting mechanism for a focusing plate which is adapted to adjust the position of a focusing plate in a single lens reflex camera.
2. Description of the Related Art
In a single lens reflex camera, object image carrying light transmitted through a photographing lens and reflected by a quick return mirror is converged onto a focusing plate to form an object image which can be viewed through a pentagonal prism (or pentagonal mirror) and an ocular (eyepiece) system.
The focusing plate can be provided with an indicia (indication area) which represents a predetermined area on the image surface, such as a photometering area or an object distance measuring area, etc. The indicia can be viewed through the view finder for example when a composition is determined.
The focusing plate must be placed in a position which is optically conjugate with the film surface. To this end, a focusing mechanism is provided to adjust the position of the focusing plate in the optical axis direction.
It is known for a camera to have an interchangeable focusing plate incorporated to vary a pattern of the indication area. It is also known for a camera to have an adjusting mechanism to adjust the position of the focusing plate in the optical axis direction and a support mechanism which detachably holds the focusing plate are provided. However, since the adjusting mechanism and the support mechanism are independent from one another, the focusing plate cannot be always stably held. Moreover, it is difficult to attach or detach the focusing plate at a certain adjusted position of the focusing plate.
A known focusing plate adjusting mechanism and attaching/detaching mechanism will be discussed below with reference to FIGS. 8 through 10.
As can be seen in FIG. 10, the focusing plate 150 is generally rectangular and is pressed at the three sides thereof onto an upper surface of a generally U-shaped adjusting frame 152 by means of a focusing plate support frame 151.
The adjusting frame 152 is secured at the four corners thereof to a camera body 160 by means of set screws 153. A predetermined thickness of spacer 154 is inserted between the adjusting frame 152 and the camera body 160. The thickness of the spacer 154 is appropriately selected to adjust the position of the focusing plate 150 in the optical axis direction.
The camera body 160 is provided at the front end thereof with a pair of posts 161 which are in turn provided on the upper ends thereof with leaf springs 162 secured thereto by means of set screws 163, as may be seen in FIGS. 8 and 9.
When the focusing plate 150 is attached to the camera body, the focusing plate support frame 151 is moved and pressed forward so that the upper surface of the front end of the support frame 151 abuts against the lower surfaces of the leaf springs 162. Consequently, the support frame 151 is biased downward by the leaf springs 162, so that the edge of the lower surface of the focusing plate 150 is pressed onto and firmly supported by the upper surface of the adjusting frame 152.
Namely, the focusing plate support frame 151 is inserted at its front end in a gap 164 defined between the leaf springs 162 and the adjusting frame 152. The gap 164 (i.e., the distance between the leaf springs 162 and the front end of the adjusting frame 152) varies depending on the axial position of the adjusting frame 152. Specifically, if a thick spacer 154 is used, the axial length of the gap 164 is reduced. Conversely, if a thin spacer 154 is used, the axial length of the gap 164 is increased.
If the gap 164 is small, it is difficult to insert the front end of the support frame 151 between the leaf springs 162 and the front end of the support frame 152. Namely, the attachment of the focusing plate 150 is made difficult. If the gap 164 is large, the spring force of the leaf springs 162 to press the focusing plate 150 onto the adjusting frame 152 is insufficient, so that it is impossible to stably or firmly hold the focusing plate 150.
Since the adjustment of the axial position of the focusing plate 150 is carried out by changing the thickness of the spacer 154 (exchanging the spacers having different thicknesses), the focusing plate 150 can be translated in the axial direction only. Namely, for example, no fine adjustment of the inclination angle of the focusing plate 150 with respect to the optical axis can be carried out.
Moreover, since the almost entire upper surface of the adjusting frame 152 is in surface contact with the lower surface of the focusing plate 150, if the adjusting frame 152 is distorted, i.e. does not lie on a flat plane, the focusing plate 150 cannot be stably held and there is an error of the adjusted position.